The present invention relates to apparatuses for detecting a rotational angle, and, more particularly, to apparatuses for detecting the rotational angle of a steering wheel of a vehicle.
FIG. 6 shows a prior art rotational angle detecting apparatus 51. The rotational angle detecting apparatus 51 includes a first detecting piece 54 and a second detecting piece 55 that oppose each other. A rotary disk 53 is located between the first and second detecting pieces 54, 55 and is secured to a steering shaft 52. An opening group 56 is located at an outer circumferential portion of the rotary disk 53. The opening group 56 is used for detecting a relative angle of the steering shaft 52, or the angle by which the rotary disk 53 is rotated relative to a reference position. The opening group 56 is located along a circle the center of which corresponds to point O. The opening group 56 includes a plurality of openings 56a that extend through the rotary disk 53 and are separated from adjacent openings 56a at equal intervals. As shown in FIG. 8, the circumferential dimension W1 of each opening 56a is equal to the interval W2 between adjacent openings 56a. As shown in FIGS. 6 and 7, an opening 57 extends through the rotary disk 53 and is located radially inward from the opening group 56. That is, the opening 57 is located on a circle that is coaxial with the circle along which the openings 56a are located. The opening 57 is used for determining the reference position of the rotary disk 53.
The first detecting piece 54 includes a first light emitting element 58a and a second light emitting element 59a. The first and second light emitting elements 58a, 59a are located at positions corresponding to a path defined by the opening group 56 when the rotary disk 53 rotates. The first piece 54 includes a third light emitting element 60a located at a position corresponding to a path defined by the opening 57 when the rotary disk 53 rotates. The second detecting piece 55 includes a first light receiving element 58b and a second light receiving element 59b, which oppose the first light emitting element 58a and the second light emitting element 59a, respectively. The second detecting piece 55 includes a third light receiving element 60b that opposes the third light emitting element 60a. 
The first light emitting element 58a and the first light receiving element 58b form a first detector 58. In the same manner, the second light emitting element 59a and the second light receiving element 59b form a second detector 59. The first and second detectors 58, 59 form a rotational angle sensor 61 that detects the angle by which the steering shaft 52, or the rotary disk 53, is rotated. Further, the third light emitting element 60a and the third light receiving element 60b form a reference position sensor 60. The reference position sensor 60 detects that the rotary disk 53 is located at the reference position.
The first detector 58 of the angle sensor 61 generates a binary code depending on whether or not the rotary disk 53 blocks the first light emitting element 58a from the first light receiving element 58b. Likewise, the second detector 59 of the angle sensor 61 generates a binary code depending on whether or not the rotary disk 53 blocks the second light emitting element 59a from the second light receiving element 59b. Thus, the rotational angle sensor 61 generates a two-bit code depending on whether or not each detector 58, 59 faces any opening 56a of the opening group 56.
The first and second detectors 58, 59 are located relative to each other such that the phase of the binary code emitted by the second detector 59 is offset from the phase of the binary code emitted by the first detector 58 by a quarter cycle. More specifically, if the first and second detectors 58, 59 both face openings 56a, as shown in FIG. 8(a), the rotational angle sensor 61 generates a two-bit code xe2x80x9c1xe2x80xa21xe2x80x9d. If the rotary disk 53 rotates from this state in a direction indicated by the arrow of FIG. 8(a) to block the first light emitting element 58a from the first light receiving element 58b, as shown in FIG. 8(b), the rotational angle sensor 61 generates a two-bit code xe2x80x9c0xe2x80xa21xe2x80x9d. If the rotary disk 53 further rotates in the same direction to block both the first and second light emitting elements 58a, 59a from the associated light receiving elements 58b, 59b, as shown in FIG. 8(c), the rotational angle sensor 61 generates a two-bit code xe2x80x9c0xe2x80xa20xe2x80x9d. If the rotary disk 53 further rotates in the same direction to block the second light emitting element 59a from the second light receiving element 59b, as shown in FIG. 8(d), the rotational angle sensor 61 generates a two-bit code xe2x80x9c1xe2x80xa20xe2x80x9d. If the rotary disk 53 further rotates in the same direction, the first and second detectors 58, 59 both face corresponding openings 56a, thus restoring the state of FIG. 8(a). In other words, when the rotary disk 53 rotates in the direction indicated by the arrows of FIGS. 8(a) to 8(d), the rotational angle sensor 61 successively generates two-bit codes xe2x80x9c1xe2x80xa21xe2x80x9d, xe2x80x9c0xe2x80xa21xe2x80x9d, xe2x80x9c0xe2x80xa20xe2x80x9d, and xe2x80x9c1xe2x80xa20xe2x80x9d in this order in a repeated manner. If the rotary disk 53 rotates in an opposite direction, the angle sensor 61 successively generates the two-bit codes in the opposite order.
Accordingly, the rotational direction of the rotary disk 53 is determined in accordance with the order in which the angle sensor 61 generates the two-bit codes. Further, the rotational angle of the rotary disk 53 is also detected in accordance with the angle of motion required to change the two-bit code. That is, if the rotational angle sensor 61 is configured to generate a two-bit code each time the rotational angle of the rotary disk 53 varies by one degree, the resolution of the rotational angle detecting apparatus 51 is one degree.
The reference position sensor 60 generates a binary code depending on whether or not the rotary disk 53 blocks the third light emitting element 60a from the third light receiving element 60b. More specifically, if the opening 57 is located between the third light emitting element 60a and the third light receiving element 60b, the reference position sensor 60 generates a binary code xe2x80x9c1xe2x80x9d. Otherwise, the reference position sensor 60 generates a binary code xe2x80x9c0xe2x80x9d.
In the rotational angle detecting apparatus 51, which is constructed as described above, operation is initiated by the reference position sensor 60. More specifically, if the reference position sensor 60 detects that the opening 57 is located between the third light emitting element 60a and the third light receiving element 60b, the corresponding angle at which the rotary disk 53 is rotated is defined as the reference angle (which is, for example, zero degrees). A memory of the rotational angle detecting apparatus 51 is thus initialized. Subsequently, the rotational angle sensor 61 generates a two-bit code depending on whether or not each detector 58, 59 faces any opening 56a. The rotational angle detecting apparatus 51 thus computes the rotational angle of the rotary disk 53 with respect to the reference angle in accordance with the two-bit code generated by the rotational angle sensor 61.
However, in the rotational angle detecting apparatus 51, the reference angle of the rotary disk 53 is determined only in accordance with the position of the opening 57. Thus, the memory of the rotational angle detecting apparatus 51 may not be initialized unless the rotary disk 53 is rotated at substantially 360 degrees. This complicates the initialization.
Accordingly, it is an objective of the present invention to provide a rotational angle detecting apparatus that easily initializes a memory regarding a rotational angle of a rotary disk.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, the invention provides a rotational angle detecting apparatus. A relative rotational angle of a rotary body is measured with respect to a reference angle. The apparatus includes a disk, a first group of openings, a first detecting element, a computing means, a second group of openings, a second detecting element, a memory, and a determining means. The disk is coaxial with the rotary body. The disk rotates integrally with the rotary body. The first group of openings includes a plurality of openings arranged about the entire circumference of the disk and spaced from one another by a predetermined pitch. The openings of the first group are used for detecting the relative rotational angle. The first detecting element detects openings of the first group and generates a first binary code. The computing means computes the relative rotational angle of the rotary body in accordance with the first binary code. The second group of openings includes a plurality of openings arranged concentrically with the first group. The openings of the second group are located relative to one another in accordance with a predetermined arrangement. The second detecting element detects openings of the second group and generates a second binary code. The memory stores a combination of the first binary code and the second binary code in correspondence with angle data. The determining means determines the reference angle in accordance with the combination of the first and second binary codes stored by the memory when the first detecting element generates a predetermined binary code.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.